The elucidation of mechanisms of synthesis, processing and transport of viral components at the molecular level will help us to understand problems of defective viral assembly implicated in some neurological diseases. To study the biosynthesis of viral macromolecular components, we prepared monoclonal antibodies reacting with 1) different sites of polypeptides of two negative stranded RNA viruses (vesicular stomatitis virus and measles virus), 2) polyclonal antibodies made against synthetic peptides corresponding in sequence to portions of the viral polypeptides, and 3) genes coding for some of the viral polypeptides either cloned into convenient expression vectors or labeled to detect viral mRNAs in cells by in situ hybridization. With electron microscopy and immunolabeling techniques for viral proteins, we have determined how the nucleocapsid interacts with the viral glycoprotein which is integrated in the plasma membrane and how the M protein may induce nucleocapsid coiling during viral maturation. Studies with a temperature-sensitive mutant in the M protein of VSV indicate that normal transport of the M protein to the membrane and normal conformation are necessary for nucleocapsid coiling and viral budding to occur. Cellular fractionation studies and gel analyses show that, at nonpermissive temperatures, abnormal M proteins accumulate around the perinuclear membranes. Using the deep-etch replica analyses on fast frozen cells, the M protein aggregates of measles were visualized for the first time on the inner face of the membrane. Abnormal interactions between measles nucleocapsids and actin may lead to defective budding.